Method of Backflow Prevention In a Gas Delivery Line

Abstract

A method for preventing flow reversal due to an upstream pressure drop in a natural gas supply line. The method, using a unique system configuration, includes setting a threshold low pressure for the upstream gas flow, continually sensing the upstream pressure, activating a trigger valve when the upstream pressure falls below the threshold low pressure, and closing a control valve in response to the trigger valve to prevent reversal of flow in the gas supply line.

What is claimed is: 
     
         1 . A method for preventing flow reversal in a natural gas supply line due to an upstream pressure drop, the method comprising:
 throttling a gas pressure through the natural gas supply line, using a worker regulator positioned downstream of a monitor control valve, to maintain a downstream delivery side gas flow pressure;   setting a threshold low pressure (X) for gas flow upstream of the monitor control valve;   continually sensing an actual gas flow pressure, P 1 , at a point upstream of the monitor control valve;   continually sensing an actual gas flow pressure, P 2 , at a point downstream of the monitor control valve, wherein the threshold low pressure (X) is set to be greater than the actual gas flow pressure (P 2 ); and   closing the monitor control valve when the actual flow pressure upstream of the monitor control valve, P 1 , falls below the threshold low pressure (X) to prevent reversal of flow in the gas supply line.   
     
     
         2 . The method of  claim 1 , further comprising re-opening the closed monitor control valve when the actual flow pressure upstream of the monitor control valve, P 1 , exceeds the threshold low pressure (X). 
     
     
         3 . The method of  claim 1 , wherein the monitor control valve is a rotary control valve. 
     
     
         4 . The method of  claim 1 , wherein the monitor control valve is a linear control valve. 
     
     
         5 . The method of  claim 1 , further comprising throttling gas flow with the monitor control valve when actual gas flow pressure at a point downstream of the monitor control valve, P 2 , exceeds a predetermined pressure value. 
     
     
         6 . The method of  claim 5 , wherein the threshold low pressure (X) is greater than 220 psig. 
     
     
         7 . A gas supply line control system comprising:
 a worker regulator for maintaining a downstream pressure in a gas supply line at a target pressure;   a monitor control valve upstream of the worker regulator and having an inlet, an outlet, and a mechanism for moving within a range between and including a fully open position and a fully closed position to control gas flow in the gas supply line, wherein gas flows from upstream to the inlet, to downstream through the outlet;   a first pressure sensor for determining a flow pressure upstream of the monitor control valve inlet; and   a trigger valve responsive to the sensor;   wherein the trigger valve triggers the monitor control valve mechanism to fully close to prevent reverse gas flow when the upstream flow pressure falls below a first predetermined value.   
     
     
         8 . The gas supply line control system of  claim 7 , further comprising a second pressure sensor for determining a flow pressure downstream of the monitor control valve outlet. 
     
     
         9 . The gas supply line control system of  claim 8 , wherein the monitor control valve throttles the gas flow when the second pressure sensor detects a downstream pressure exceeding a second predetermined value. 
     
     
         10 . The gas supply line control system of  claim 9 , wherein the second predetermined value is 160 psig.